[analyzer] Consider array subscripts to be interesting lvalues.

Static analyzer has a mechanism of clearing redundant nodes when
analysis hits a certain threshold with a number of nodes in exploded
graph (default is 1000). It is similar to GC and aims removing nodes
not useful for analysis. Unfortunately nodes corresponding to array
subscript expressions (that actively participate in data propagation)
get removed during the cleanup. This might prevent the analyzer from
generating useful notes about where it thinks the data came from.

This fix is pretty much consistent with the way analysis works
already. Lvalue "interestingness" stands for the analyzer's
possibility of tracking values through them.

Differential Revision: https://reviews.llvm.org/D78638